The components in the hot sections of gas turbine engines are exposed to increasingly harsh operating environments. Such components during their operation rely on protective coatings that can increase their durability and reliability.
Components in the gas turbine engines are typically made from superalloy materials. Although such superalloy materials impart suitable high-temperature mechanical properties, they possess insufficient environmental resistance. Consequently, superalloy components, such as combustors, high pressure blades, shrouds and vanes, have been applied with diffusion aluminide coatings for protection against oxidation and corrosion attacks at high temperatures. These aluminide coatings are used singularly or as bond coats for the thermal barrier coating (TBC) systems. Aluminide coatings are sufficiently rich in aluminum (Al) to form a protective and thermally grown oxide scale, known as alpha alumina. They are generally produced by enriching the superalloy surface with aluminum by various methods such as pack cementation, vapor phase process or chemical vapor deposition.
Further on-going improvements in gas turbine engine performance has required even higher operating efficiency and reduced emission, which necessitated the development for improved aluminide coatings capable of withstanding higher operating temperatures. One strategy recognized and accepted in the industry to improve the performance of aluminide coatings is to incorporate a small amount of so-called reactive element (RE), such as hafnium, zirconium, yttrium, lanthanum, or cerium, to produce a reactive element (RE)-doped aluminide coating that can enhance the high-temperature oxidation resistance of metallic alloys and coatings. Furnace cycle testing (“FCT”) as used herein is used to quantify the oxidation resistance of an aluminide coating or a thermal barrier coating (“TBC”) system with an aluminide bond coat. Typical reactive elements include, but are not limited to hafnium, zirconium, yttrium, and lanthanide series of elements in the periodic table. The minor additions of such reactive elements are known to significantly enhance the high-temperature oxidation resistance of the aluminide coatings during thermal cycling.
FCT oxidation resistance of the RE-doped aluminide coating has been recognized to be sensitive to concentrations of RE in the coating. In this regard, the so-called FCT performance can range from no improvement up to about 4× improvement over conventional diffusion aluminide coatings without any RE incorporated therein.
To date, various processes have been explored to form reactive element-doped aluminide coatings. The phrase “reactive element-doped aluminide coating” as used herein and throughout the specification refers to the diffusion aluminide coatings that include at least one of the reactive elements selected from the group of hafnium, zirconium, yttrium, lanthanum, and/or cerium, the presence of which can enhance the performance of the conventional diffusion aluminide coating under corrosive environments. Such processes include pack cementation, vapor phase processes, or chemical vapor deposition (CVD) processes, in which the additions of RE donor materials into the aluminizing coating retort are employed. However, these processes have numerous drawbacks whereby repeatability of RE concentration cannot be attained so as to produce consistent and acceptable FCT performance.
Thus, there exists a need for a simpler, more economical and controllable method to produce RE-doped aluminide coatings on surfaces, including part surfaces having complex geometries such as high-pressure turbine blades, vanes, and shrouds in the gas turbine engines. Other advantages and applications of the present invention will become apparent to one of ordinary skill in the art.